There has been conventionally known an optical sorting machine which sorts a raw material of grains such as rice, wheat, pulses and nuts, resin pieces such as pellets and beads, fine articles such as medicines, ores and whitebait, or other granular objects into non-defective products and defective products, and removes foreign objects or the like mixed into the raw material.
This type of optical sorting machine illuminates granular objects falling along a predetermined trajectory from an end portion of a transfer passage, detects defective products or foreign objects included in the raw material upon receiving reflected light or the like from the granular objects, and removes the detected defective products or foreign objects, thereby sorting the granular objects.
While the optical sorting machine removes the defective products or the foreign objects by, for example, ejecting air to the granular objects falling along the predetermined trajectory, non-defective products existing near the defective products are caught and removed together.
Thus, there is known a method for sorting a raw material of granular objects into non-defective products and defective products in two stages by providing two sets of sorting units in parallel so as to improve the collection rate of non-defective products in the optical sorting machine (e.g., see Patent Literature 1).
FIG. 6 shows an explanatory view of a procedure for sorting non-defective products and defective products in the optical sorting machine described in Patent Literature 1.
In the optical sorting machine described in Patent Literature 1, a raw material of granular objects fed to a primary tank 121a is sorted into non-defective products and defective products in a primary sorting unit 103a. The granular objects sorted into the defective products in the primary sorting unit 103a are conveyed to a first elevator 105a through a passage 104a, conveyed to a secondary tank 121b by the first elevator 105a, and thereafter sorted into non-defective products and defective products in a secondary sorting unit 103b. The granular objects sorted into the non-defective products in the secondary sorting unit 103b are conveyed to a second elevator 105b through a passage 104b, conveyed to the primary tank 121a by the second elevator 105b, and sorted again into non-defective products and defective products in the primary sorting unit 103a. 
In accordance with the optical sorting machine described in Patent Literature 1, it is possible to collect non-defective products caught in air for removing defective products in the primary sorting unit 103a and mixed into the defective products, and thereby improve the collection rate of non-defective products.
The optical sorting machine described in Patent Literature 1, however, has a problem that the machine is increased in size and cost since it is necessary to provide two sets of sorting units.
The optical sorting machine described in Patent Literature 1 also has a problem that, while many non-defective products caught in the air for removing defective products are mixed into the defective products conveyed to the secondary tank 121b, the secondary sorting unit cannot efficiently collect the non-defective products because its processing capacity is limited.